Conventionally, as shown in FIG. 5, a vehicle seat frame is known, which includes a seat cushion frame 110 supporting a seat cushion 90, a seat back frame 120 supporting a seat back 91, a recliner 140 rotatably connecting the seat back 91 relative to the seat cushion 90 and adjusting the rotation angle thereof and a shoulder belt anchor 130 provided at an upper end of the seat back frame 120. Back side frames 121 and 122 are provided at both sides of the seat back frame 120 and a hollow square prism shaped back center frame 123 is provided at the central portion thereof. Further, cushion side frames 111 and 112 are provided at both sides of the seat cushion frame 110 and a cushion center frame 113 is provided at the central portion thereof. An upper arm 124 formed with a plank is connected to the under portion of the back center frame 123 through bolt means and a lower arm 114 formed with a plank is connected to the upper portion of the cushion center frame 113 through bolt means. The recliner 140 is provided between the upper arm 124 and lower arm 114. The recliner 140 rotatably connects the upper and lower arms 124 and 114.
Further, a retractor 150 is attached to the seat back frame 120 and one end of a seat belt 151, which is structured with a shoulder belt portion 151a and a lap belt portion 151b, is retracted into the retractor 150. The other end of the seat belt 151 is fixed to an anchor plate 115 attached to the cushion center frame 113 and the middle portion of the seat belt 151 is inserted into a tongue plate 152. This tongue plate 152 is detachably attached to a buckle 116 attached to the cushion side frame 111. It is noted that the vehicle seat frame is a seat frame for two occupants and an explanation of the other seat belt for the other person is omitted. According to this vehicle seat frame, since the retractor 150 is attached to the seat back frame 120, when the seat belt 151 is locked upon the vehicle being suddenly decelerated, a large load is applied on the back center frame 123 of the seat back frame 120 through the shoulder belt anchor 130. This may lead to a deformation of the back center frame 123.
Another conventional vehicle seat frame is proposed which is disclosed in the Patent Document 1. As shown in FIG. 6, the back center frame (tower frame) 123 of the vehicle seat frame according to the Patent Document 1 is formed by welding the side brackets 123a and 123b having approximately U-shape (or reverse C-shape) in cross section and an elongated reinforcement plate (insert brace) 123c is inserted between the side brackets 123a, 123b. According to this vehicle seat frame, since the elongated reinforcement plate (insert brace) 123c is inserted between the both side brackets 123a and 123b, even when the vehicle is suddenly decelerated and a large load is applied on the back center frame (tower frame) 123, the back center frame (tower frame) would not be easily deformed.
Patent Document 1: JA 2002-283891 A